Network Working Group H. Schulzrinne

Network Working Group H. Schulzrinne
Request for Comments: 3326 Columbia University
Category: Standards Track D. Oran
Cisco
G. Camarillo
Ericsson
December 2002

The Reason Header Field for the Session Initiation Protocol (SIP)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

For creating services, it is often useful to know why a Session
Initiation Protocol (SIP) request was issued. This document defines
a header field, Reason, that provides this information. The Reason
header field is also intended to be used to encapsulate a final
status code in a provisional response. This functionality is needed
to resolve the "Heterogeneous Error Response Forking Problem", or
HERFP.

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RFC 3326 The Reason Header Field for SIP December 2002

Table of Contents

1. Introduction ............................................... 2
1.1. Terminology ................................................ 3
2. The Reason Request Header Field ............................ 3
3. Examples ................................................... 4
3.1. Call Completed Elsewhere ................................... 4
3.2. Refusing an Offer that Comes in a Response ................. 4
3.3. Third Party Call Control ................................... 5
3.4. ISUP interworking .......................................... 5
4. IANA Considerations ........................................ 6
5. Security Considerations .................................... 6
6. Acknowledgments ............................................ 7
7. Authors' Addresses ......................................... 7
8. Normative References ....................................... 7
9. Full Copyright Statement ................................... 8

1. Introduction

The same SIP [1] request can be issued for a variety of reasons. For
example, a SIP CANCEL request can be issued if the call has completed
on another branch or was abandoned before answer. While the protocol
and system behavior is the same in both cases, namely, alerting will
cease, the user interface may well differ. In the second case, the
call may be logged as a missed call, while this would not be
appropriate if the call was picked up elsewhere.

Third party call controllers sometimes generate a SIP request upon
reception of a SIP response from another dialog. Gateways generate
SIP requests after receiving messages from a different protocol than
SIP. In both cases the client may be interested in knowing what
triggered the SIP request.

SIP responses already offer a means of informing the user of why a
request failed. The simple mechanism in this document accomplishes
something roughly similar for requests.

An INVITE can sometimes be rejected not because the session
initiation was declined, but because some aspect of the request was
not acceptable. If the INVITE forked and resulted in a rejection,
the error response may never be forwarded to the client unless all
the other branches also reject the request. This problem is known as
the "Heterogeneous Error Response Forking Problem", or HERFP. It is
foreseen that a solution to this problem may involve encapsulating
the final error response in a provisional response. The Reason header
field is a candidate to be used for such encapsulation.

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RFC 3326 The Reason Header Field for SIP December 2002

Initially, the Reason header field defined here appears to be most
useful for BYE and CANCEL requests, but it can appear in any request
within a dialog, in any CANCEL request and in any response whose
status code explicitly allows the presence of this header field.

Note that the Reason header field is usually not needed in responses
because the status code and the reason phrase already provide
sufficient information.

Clients and servers are free to ignore this header field. It has no
impact on protocol processing.

1.1 Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[2] and indicate requirement levels for compliant SIP
implementations.

2. The Reason Header Field

The Reason header field MAY appear in any request within a dialog, in
any CANCEL request and in any response whose status code explicitly
allows the presence of this header field. The syntax of the header
field follows the standard SIP parameter syntax.

Reason = "Reason" HCOLON reason-value *(COMMA reason-value)
reason-value = protocol *(SEMI reason-params)
protocol = "SIP" / "Q.850" / token
reason-params = protocol-cause / reason-text
/ reason-extension
protocol-cause = "cause" EQUAL cause
cause = 1*DIGIT
reason-text = "text" EQUAL quoted-string
reason-extension = generic-param

The following values for the protocol field have been defined:

SIP: The cause parameter contains a SIP status code.

Q.850: The cause parameter contains an ITU-T Q.850 cause value
in decimal representation.

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RFC 3326 The Reason Header Field for SIP December 2002

Examples are:

Reason: SIP ;cause=200 ;text="Call completed elsewhere"
Reason: Q.850 ;cause=16 ;text="Terminated"
Reason: SIP ;cause=600 ;text="Busy Everywhere"
Reason: SIP ;cause=580 ;text="Precondition Failure"

Proxies generating a CANCEL request upon reception of a CANCEL from
the previous hop that contains a Reason header field SHOULD copy it
into the new CANCEL request.

In normal SIP operation, a SIP status code in a response provides the
client with information about the request that triggered the
response, the session parameters, or the user. For example, a 405
(Method not allowed) response indicates that the request contained an
unsupported method. A 488 (Not Acceptable Here) indicates that the
session parameters are unacceptable and a 486 (Busy Here) provides
information about the status of the user.

Any SIP status code MAY appear in the Reason header field of a
request. However, status codes that provide information about the
user and about session parameters are typically useful for
implementing services whereas status codes intended to report errors
about a request are typically useful for debugging purposes.

A SIP message MAY contain more than one Reason value (i.e., multiple
Reason lines), but all of them MUST have different protocol values
(e.g., one SIP and another Q.850). An implementation is free to
ignore Reason values that it does not understand.

3. Examples

This section contains a number of examples that illustrate the use of
the Reason header field.

3.1 Call Completed Elsewhere

A proxy forks an INVITE request and one of the branches returns a 200
(OK). The forking proxy includes this status code in a Reason header
field in the CANCEL request that it sends to the rest of the
branches.

3.2 Refusing an Offer that Comes in a Response

A client sends an empty INVITE and receives an unacceptable offer in
a 200 (OK) response. The client sends an ACK with a correctly
formatted answer and immediately sends a BYE to terminate the

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RFC 3326 The Reason Header Field for SIP December 2002

session. The client includes a 488 (Not Acceptable Here) status code
in a Reason header field.

3.3 Third Party Call Control

The third party call controller of figure 1 tries to establish a
session between A and B. However, user B is busy. The controller
sends a BYE with the status code 486 (Busy Here) in a Reason header
field.

A Controller B
| INV no SDP | |
|<------------------| |
| | |
| 200 SDP A1 | |
|-----------------> | |
| | |
| ACK SDP held | |
|<------------------| |
| | |
| | INV no SDP |
| |----------------->|
| | |
| | 486 Busy Here |
| |<-----------------|
| | |
| | ACK |
| |----------------->|
| BYE (486) | |
|<------------------| |
| | |
| 200 OK | |
|-----------------> | |
| | |

Figure 1: Third Party Call Control

3.4 ISUP interworking

The PSTN gateway of figure 2 generates an INVITE that has to be
CANCELed when a REL (release) message is received from the ISUP side.
The CANCEL request contains the Q.850 cause value (16 Normal Call
Clearing) of the REL message.

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RFC 3326 The Reason Header Field for SIP December 2002

A Gateway B
| IAM | |
|-----------------> | |
| | INVITE |
| |----------------->|
| | |
| | 100 Trying |
| |<-----------------|
| REL (16) | |
|-----------------> | |
| | CANCEL (Q.850 16)|
| |----------------->|
| | 200 OK |
| |<-----------------|

Figure 2: ISUP Interworking

4. IANA Considerations

This document defines a new SIP header field, "Reason", according to
Section 27 of RFC 3261.

Protocol values (and their associated protocol cause) to be used with
this header field are registered by the IANA into a new sub-registry
under http://www.iana.org/assignments/sip-parameters, labeled "Reason
Protocols". Reason protocols MUST refer to either an ITU-T
Recommendation number, an IETF protocol name or the recognized
protocol identifier from another standardization organization.
Protocol cause describes the source of the 'cause' field in the
Reason header field.

The only entries in the registry for the time being are:

Protocol Value Protocol Cause Reference
-------------- --------------- -----------
SIP Status code RFC 3261
Q.850 Cause value in decimal ITU-T Q.850
representation

5. Security Considerations

Spoofing or removing the Reason header field from a response in a
HERFP scenario can make it impossible for a client to update properly
its previous request, making therefore session establishment
impossible. Thus, it is RECOMMENDED that this header field is
protected by a suitable integrity mechanism.

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RFC 3326 The Reason Header Field for SIP December 2002

properly its previous request, making therefore session establishment
impossible. Thus, it is RECOMMENDED that this header field is
protected by a suitable integrity mechanism.

6. Acknowledgments

Jonathan Rosenberg, Rohan Mahy and Vijay K. Gurbani provided helpful
comments and suggestions.

8. Normative References

[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.

[2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels," BCP 14, RFC 2119, March 1997.

7. Authors' Addresses

Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA

EMail: [email protected]

David R. Oran
Cisco Systems, Inc.
Acton, MA
USA

EMail: [email protected]

Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland

EMail: [email protected]

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RFC 3326 The Reason Header Field for SIP December 2002

9. Full Copyright Statement

Copyright (C) The Internet Society (2002). All Rights Reserved.

This document and translations of it may be copied and furnished to
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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